The present invention relates to ion sources in mass spectrometers.
A mass spectrometer analyzes masses of molecules, such as ions, and includes an ion source and one or more mass analyzers. In the ion source, particles are ionized and extracted from a sample that includes analyte molecules, i.e., molecules to be analyzed. The ions are transported to one or more mass analyzers, e.g., time-of-flight, ion trap or multipole analyzers, that analyze the ions based on their mass-to-charge ratio.
In the ion source, the particles can be ionized by a variety of techniques, e.g., using static electric fields or laser, electron, or other particle beams. For example, in a laser desorption ionization (LDI) technique, a laser beam, typically delivered in pulses, is focused on a surface of a sample from which the focused laser ablates particles. The laser's wavelength can be adjusted to selectively heat and ablate analyte molecules in the sample. Near the sample surface, the ablated particles form a “plume” of particles, including ionized analyte molecules and, for large analyte molecules, fragments of the molecules.
Fragmentation can be avoided using matrix-assisted laser desorption ionization (“MALDI”) techniques. In MALDI, analyte molecules are embedded in a matrix, which typically includes small organic molecules. For example, the analyte and matrix molecules can be mixed in a solvent, which is later removed, e.g., by drying the solution. In this technique, the laser's energy is absorbed mainly by matrix molecules that heat up and vaporize into a particle plume. The matrix molecules drag along the analyte molecules into the plume where analyte molecules become ionized.
From the particle plume created by the laser beam, analyte ions can be guided to a mass analyzer, e.g., by multipole rod assemblies, such as a quadrupole rod assembly in which four parallel rods create a quadrupole electric field to guide the ions from the sample surface to the analyzer. To avoid distortions of the quadrupole field, the laser beam irradiates the sample through a gap between the quadrupole rods.